CFB 2043 – ORGANIC CHEMISTRY

AROMATIC COMPOUNDS
LECTURE 7

TS DR MOHD DZUL HAKIM WIRZAL

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COURSE OUTCOME
This lecture address the following course outcome

Describe the special stability of aromatic compounds and how this

3 affects reactivity.

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LEARNING OUTCOME
At the end of this lecture, students should be able to:

To explain the stability of benzene and other aromatic compounds.

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2 To name benzene’s derivatives compounds from the given structure.

3 To determine the aromaticity of a given compound.

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 PART 1
OCCURRENCE AND HISTORY
AROMATICITY
▪ Aromatic was used to described some fragrant compounds in early 19th century.

▪ Not correct: later they are grouped by chemical behavior — unsaturated

compounds that undergo substitution rather than addition.

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AROMATICITY (CONT.)
▪ Aromatic is unsaturated compounds that undergo substitution rather than
addition.

▪ For example, benzene does not react with Br2 to yield an addition product.
Instead, in the presence of a Lewis acid, bromine substitutes for a hydrogen
atom, thus yielding a product that retains the benzene ring.

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AROMATICITY (CONT.)
▪ Aromaticity has to do with the unusual stability of the compound benzene and
its derivatives, as well as certain other unsaturated ring compounds.
▪ The structures of these compounds are often shown to contain double bonds,
but they do not actually behave like double bonds.

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AROMATIC COMPOUNDS - CAPSAICIN

Capsaicin is responsible for the characteristic

spicy flavor of jalapeño and habañero peppers.
Although it first produces a burning sensation
on contact with the mouth or skin, repeated
application desensitizes the area to pain.

Capsaicin is an aromatic compound because it

contains a benzene ring. In this chapter, we
learn about the characteristics of aromatic
compounds like capsaicin.
AROMATIC COMPOUNDS – BENZO[A]PYRENE

Benzo[a]pyrene, produced by the incomplete oxidation of organic

compounds in tobacco, is found in cigarette smoke.
AROMATIC COMPOUNDS - SPICES
▪ Organic compounds with pleasant smells were originally classified as aromatic
compounds.
▪ Many of these contain a benzene ring in their structure and nowadays an
aromatic compound Is one structurally derived from benzene, C6H6.
HISTORY OF BENZENE: DISCOVERY
1825 Isolated by Michael Faraday who deduced its empirical formula was CH

"When we consider the magnitude and extent of his discoveries and their influence on the
progress of science and of industry, there is no honour too great to pay to the memory of
Faraday, one of the greatest scientific discoverers of all time“ – Albert Einstein.

1834 The German scientist, Eilhard Mitscherlich determined that molecular

weight of Benzene was 78 and its formula was C6H6.

Mitscherlich's grave in Berlin

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HISTORY OF BENZENE: CHEMICAL STRUCTURES?

1865 Kekulé - Dreams of Molecules & Benzene Structure

This structure at once explained a huge number of phenomena
connected with benzene. Organic chemists were now able to go
ahead and make thousands of derivatives of benzene for
industrial, medical and other uses.

This discovery was so important that its 25th

anniversary was celebrated in great style in 1890
by the German Chemical Society.
Said an excited Kekulé to his colleagues, “Let us
learn to dream!”

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HISTORY OF BENZENE: HERE WE GO…
The Kekulé structure of Benzene as a hexagonal molecule with
alternate double and single bonds. Each carbon atom is attached to
one hydrogen atom. This model was later modified to one with two
isomers, rapidly interconverting to one another.

The chemical name for the Kekulé

structure of benzene is cyclohexa-
1,3,5-triene, after the positions of
the double bonds in the ring

1930 The chemical structures of benzene was established – Resonance &

Delocalized Structures
▪ Bond length data

0.154 nm This shows that each C-C

KEY POINT
bond in benzene ring is
intermediate between as
0.134 nm All C-C bonds are the single and double bond.
same length:0.134 nm

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HISTORY OF BENZENE: FINALLY…
▪ Thermochemical evidence

▪ The difference between thermochemical data for cyclohexa-1,3,5-triene and

KEY POINT
benzene suggests that benzene has more stable bonding than Kekulé structure.
▪ The delocalization stability of benzene of -151 kJ per mol is the difference
between the two enthalpy changes. This is extra energy that must be provided
to break the delocalized benzene ring.

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 PART 2
NOMENCLATURE OF BENZENE’ DERIVATIVES
BENZENE
The “mother” of aromatic compounds!

Four (4) ways to draw benzene

▪ NEVER draw benzene as a simple hexagon. This would be a molecule of

cyclohexane!

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BENZENE´S DERIVATIVES
Most monosubstituted aromatics are named using -benzene as the parent name
preceded by the substituent name (as a prefix; all one word):

For example:

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BENZENE’S DERIVATIVES
▪ Many monosubstituted benzenes, such as those with methyl (—CH3), hydroxy
(—OH), and amino ( –NH2) groups, have common names that you must learn,
too.

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BENZENE DERIVATIVES – MORE EXAMPLES

http://www.compoundchem.com/2014/09/01/benzene-derivatives-in-organic-chemistry/ 19
DID YOU KNOW?

IUPAC Name:
Fluorobenzene
Other Name
Phenyl fluoride
Monofluorobenzene

Fluorobenzene was first reported in 1886 by Otto. Wallach, student of Friedrich August
Kekulé, at the University of Bonn, who prepared the compound in two steps, starting also
with a phenyldiazonium salt. The diazonium chloride was first converted to its piperidine,
which in turn was cleaved using hydrofluoric acid.

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 DID YOU KNOW?
▪ In December 2014, a team of scientists announced that
the Curiosity rover reported evidence of higher
concentrations of chlorobenzene in a rock, named
"Cumberland", on Mars.

IUPAC Name:
Chlorobenzene
Other Name
benzene chloride
monochlorobenzene
Phenyl chloride
Chlorobenzol
MCB

▪ The team speculated that the chlorobenzene might have

been produced by Martian life, or by other chemical
reactions, resulting from the heating of the rock
and outgassing.

http://www.jpl.nasa.gov/news/news.php?release=2014-432; http://www.nytimes.com/2014/12/17/science/a-new-clue-in-the-search-for-life-on-mars.html?_r=0 21
DID YOU KNOW?
Other Name
Carbolic acid,
benzenol,
phenylic acid,
hydroxybenzene,
phenic acid
IUPAC Name:
Phenol

▪ Phenol was discovered in 1834 by Friedlieb Ferdinand

Runge, who extracted it (in impure form) from coal tar.
▪ Injections of phenol were used as a means of individual
execution by the Nazis during the Second World War.
▪ Approximately one gram is sufficient to cause death.
▪ One of the best known inmates to be executed with a
phenol injection in Auschwitz was St. Maximilian Kolbe,
a Polish Catholic priest who volunteered to undergo
two weeks of starvation and dehydration in the place of Maximilian Kolbe, on a West
German postage stamp, marked
another inmate. Auschwitz

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DID YOU KNOW?

Castel del Phenoli Erected in 1958 to clean phenol-contaminated waters biologically.

The rest of the abandoned coking plant were demolished in 2006, the so-called bio
towers remained as a historical technical landmark.
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NAMING BENZENE’S DERIVATIVE
▪ Alkyl substituted benzenes are named according to the
length of the carbon chain of the alkyl group.
▪ With six carbons or fewer in the alkyl chain, they are named
Alkylbenzene as ‘alkylbenzene.’
R = alkyl

Propylbenzene
Methylbenzene
Also known as toluene

Ethylbenzene Phenylethene Isopropylbenzene

Known as styrene Also known as cumene
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NAMING BENZENE´S DERIVATIVES
▪ With more than six carbons in the alkyl chain, they are named as a
‘phenylalkane,’ where the benzene ring is named as a substituent (phenyl) on the
alkane chain

6-phenylnonane 4-phenylnonane

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NAMING BENZENE’S DERIVATIVES
1. Determine the “parent” chain
2. Higher priority (small number) is given to heteroatom (non C and H)

3-phenyl-2-butanol or 3-phenyl-butan-2-ol

2-hydroxy-3-phenylbutane 2-phenyl-3-hydroxybutane

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NAMING BENZENE’S DERIVATIVES
▪ A benzene substituent is called a phenyl group, and it can be
abbreviated in a structure as “Ph-”.

▪ Therefore, benzene can be represented as PhH, and phenol would be

PhOH.

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CLASS ACTIVITY 7.1
1. Name the following benzene derivatives using IUPAC nomenclature.

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CLASS ACTIVITY: ANSWER
You should try to answer the class activity before looking to this answer scheme!

2 4 6 Parent chain: C6 = hex

5 Substituent 1: 3-ene (double bond, alkene)
1 3
Substituent 2: 5-phenyl
Full name: 5-phenyl-hex-3-ene (or 5-phenyl-3-hexene)

Point
Key
Substituent should get lower number
(1)

5 3 1 Parent chain: C6 = hex

2 Substituent 1: 3-ene (double bond, alkene)
6 4
Substituent 2: 2-phenyl
Full name: 2-phenyl-hex-3-ene (or 2-phenyl-3-hexene)

Point
Key
Phenyl at 2 instead of 5

(1)
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CLASS ACTIVITY: ANSWER
Parent chain: cyclic C6 = cyclohex
Substituent 1: 1,3-dione (di = 2, one = alkanone)
1
2 6 Substituent 2: 5-benzyl
3 5 Full name: 5-benzyl-cyclohex-1,3-dione (or 5-benzyl-
4
1,3-cyclohexadione

Alkanon have higher priority (smaller number than

Point
(2)

Key
benzyl)

Parent chain: cyclic C6 = cyclohex

Substituent 1: 1,5-dione (di = 2, one = alkanone)
1
6 2 Substituent 2: 3-benzyl
5 3 Full name: 3-benzyl-cyclohex-1,5-dione (or 3-benzyl-1,3-
4
cyclohexadione)

Point
Key
Substituent should get lower number
(2)

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CLASS ACTIVITY: ANSWER
(3) (5)

Diphenylmethane 2-amino-3-phenylpropanoic acid

(also known as phenylalanine, an amino acid)

(4) (6)

2-phenyl-propan-2-ol 2-amino-3-(4-hydroxyphenyl)propanoic acid

Or (also known as tyrosine, an amino acid)
2-phenyl-2-propanol

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DISUBSTITUTED BENZENE
▪ Disubstituted benzenes can be named in one of two ways. Each method
describes the relative positions of the two groups on the benzene ring.
• Systematic numbering of the aromatic ring.
• Using the prefixes ortho-, meta-, or para-.
▪ When numbering the ring carbons, carbon # 1 is always a substituted carbon.
▪ The substituents are listed alphabetically.

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DISUBSTITUTED BENZENE

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DISUBSTITUTED BENZENE
▪ When one of the substituents changes the base name, either o-, m-, and p- or
numbers may be used to indicate the position of the other substituent.
▪ Carbon # 1 is always the carbon bearing the substituent that changes the base
name.

Br OH
4 1 2 Cl
3

2
1
NH2
p-bromoaniline o-chlorophenol
or or
4-bromoaniline 2-chlorophenol

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DISUBSTITUTED BENZENE
▪ There are a few nonsystematic (common) names for disubstituted
benzenes that you should be familiar with:

CH3 CH3
CH3 CH3

CH3 CH3
o-xylene m-xylene p-xylene

CH3 CH3
OH CH3

OH OH
o-cresol m-cresol p-cresol

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POLYSUBSTITUTED BENZENE
▪ Common names of the monosubstituted benzenes are used as parent
names for polysubstituted aromatics when one of the substituents
changes the base name.
▪ For such rings with common names, the carbon bearing the substituent
responsible for the common name is always carbon #1.
▪ The substitutents are listed in alphabetical order.

toluene
CH3
1 Cl chloro
2
bromo
Br 5 3
4
5-bromo-2-chlorotoluene

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CLASS ACTIVITY 7.2
1. Name the following benzene derivatives using IUPAC nomenclature.

2. Draw the structure corresponding to each name:

a. Isobutylbenzene d. 4-chloro-1,2-diethylbenzene
b. m-bromoaniline e. cis-1,2-diphenylcyclohexane
c. o-dichlorobenzene f. 3-tert-butyl-2-ethyltoluene

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CLASS ACTIVITY: ANSWER
Parent base: toluene
1 6
2
Substituents: 2,4,6-trinitro 5 1
6
Full name: 2,4,6-trinitrotoluene
4 2

Point
5 3

Key
Parent base MUST be No.1
4 3

(1) (1)

Parent base: Nitrobenzene 1

2 Substituent 1: 2-methyl 6 2
3 1
Substituent 2 and 3: 3,5-dinitro
Full name: 2-methyl-1,3,5-trinitrobenzene 5 3
4 6
4
5
Point
Key

IUPAC NAME

(1)
(1)

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 PART 3
THE CRITERIA FOR AROMATICITY

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THE CRITERIA FOR AROMATICITY

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THE CRITERIA FOR AROMATICITY
1. A molecule must be cyclic

To be aromatic, each p orbital

Key Point
must overlap with p orbitals on
adjacent atoms.
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THE CRITERIA FOR AROMATICITY
2. A molecule must be planar

All adjacent p orbitals must be aligned so that the  electron density can be
delocalized.

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THE CRITERIA FOR AROMATICITY
2. A molecule must be planar

Since cyclooctatetraene is non-planar, it is not aromatic, and it undergoes

addition reactions just like those of other alkenes.

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THE CRITERIA FOR AROMATICITY
3. A molecule must be completely conjugated

Aromatic compounds must have a p orbital on every atom and each must overlap
with adjacent p orbitals.

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THE CRITERIA FOR AROMATICITY
4. A molecule must satisfy Hückel’s rule

▪ In 1931, German chemist and physicist Erich Hückel proposed a

theory to help determine if a planar ring molecule would have
aromatic properties.
▪ His rule states that if a cyclic, planar molecule has
4n+2 π electrons, it is considered aromatic. This rule would
come to be known as Hückel's Rule.

Key Point
Hückel’s Rule:
4n + 2 π electrons

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AROMATIC, ANTIAROMATIC AND NONAROMATIC
With regard to aromaticity, a compound can be classified in one of three ways:

▪ Aromatic: A cyclic, planar, completely conjugated compound with 4n + 2 

electrons.
▪ Antiaromatic: A cyclic, planar, completely conjugated compound with 4n 
electrons.
▪ Nonaromatic: A compound that lacks one (or more) of the following
requirements for aromaticity: being cyclic, planar, and completely conjugated.

A completely conjugated, monocyclic hydrocarbon is called an annulene (not

necessarily aromatic).

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AROMATIC, ANTIAROMATIC AND NONAROMATIC
Note the relationship between each compound type and a similar open-
chained molecule having the same number of  electrons.

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CLASS ACTIVITY 7.3
1. Determine if the following compounds are AROMATIC, ANTIAROMATIC, or NOT
AROMATIC

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CLASS ACTIVITY 7.3 (CONT.)
2. Determine if the following compounds are AROMATIC, ANTIAROMATIC, or NOT
AROMATIC

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THE CRITERIA FOR AROMATICITY
4. A molecule must satisfy Hückel’s rule

▪ [10]-Annulene has 10  electrons, which satisfies Hückel's rule,

but a planar molecule would place the two H atoms inside the
ring too close to each other. Thus, the ring puckers to relieve
this strain.
▪ Since [10]-annulene is not planar, the 10  electrons can’t
delocalize over the entire ring and it is not aromatic.

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AROMATICITY: MOS
Molecular Orbitals (MOs) of Benzene

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AROMATICITY: MOS (CONT.)
The Inscribed Polygon Method of Predicting Aromaticity

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AROMATICITY: MOS (CONT.)

▪ This method works for all monocyclic completely conjugated systems

regardless of ring size.
▪ The total number of MOs always equals the number of vertices of the
polygon.
▪ This method is consistent with Hückel's 4n + 2 rule, there is always one
lowest energy bonding MO that can hold two  electrons and the other
bonding MOs come in degenerate pairs that can hold a total of four 
electrons
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AROMATICITY: MOS (CONT.)
Using the inscribed polygon method for five- and seven-membered rings

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AROMATICITY: MOS (CONT.)
The Inscribed Polygon Method of Predicting Aromaticity

··

+ + ··
··

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AROMATICITY: MOS (CONT.)
For the compound to be aromatic, these MOs must be completely filled with
electrons, so the “magic numbers” for aromaticity fit Hückel’s 4n + 2 rule.

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AROMATIC IONS
Aromatic Ions
▪ The 4n + 2 rule applies to ions as well as neutral species
▪ Both the cyclopentadienyl anion and the cycloheptatrienyl cation are
aromatic
▪ The key feature of both is that they contain 6  electrons in a ring of
continuous p orbitals

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AROMATIC: HETEROCYLIC
Heterocyclic Aromatic Compounds
(contain elements other than carbon in a ring, such as N,S,O,P)

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AROMATICITY: HETEROCYLIC
Heterocyclic Aromatic Compounds
(contain elements other than carbon in a ring, such as N,S,O,P)

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CLASS ACTIVITY 7.4
Using MO and polygon approaches, determine if the following compounds are
AROMATIC or NOT.

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▪ https://www.youtube.com/watch?v=ubtvxTvd
WjA
▪ https://www.youtube.com/watch?v=BDooDi7z
Qxo
▪ https://www.youtube.com/watch?v=eK8S51L8
juo

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END OF THE CLASS
THANK YOU ☺

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